Increasingly frequently, scanning systems involve the use of high energy inspection equipment based on transmission imaging with X-radiation generated by an X-ray linear accelerator with typical beam quality of 1 MeV to 9 MeV.
Such systems are very effective at probing the structure and shape of relatively high atomic number articles but are less effective at locating the presence of low atomic number materials in sheet like configurations that are broadly perpendicular to the path of the incoming X-ray beam.
FIG. 1 is a schematic representation of a known system 2. The known system 2 comprises an x-ray source, in the form of a rotating disc x-ray source 4. An object to be scanned is shown in the form of a lorry 8. A detector 6 is arranged on the same side of the lorry as the source. The source is arranged to irradiate a single region of the object at any one time (i.e. in any one irradiation event or burst). The source produces a tightly collimated beam 10 which irradiates a point on the object 8. Scattered radiation 12 is scattered in all directions and is detected at the detector 6. The detector 6 measures the amount of radiation per irradiation event in order to provide information on the point of the object being irradiated during that event.
The inverse square law indicates that the intensity of a radiation beam reduces in proportion to the square of the distance from the source. For example, the source intensity (photons/unit area) drops by a factor of four in moving from 10 cm away from a point source of radiation to the same area 20 cm away. Therefore, the strength of the backscatter signal drops rapidly with distance into the cargo and so X-ray backscatter is predominantly a surface inspection technique.
Further, it can easily be shown that the maximum energy of a backscattered X-ray photon is 256 keV. This relatively low energy X-ray is absorbed rather readily by high density materials such as steel, again suggesting that X-ray backscatter imaging is predominantly a surface imaging method.
Therefore, what is needed is an X-ray backscatter imaging that is integrated with high intensity linear accelerator based transmission imaging in order to spatially correlate surface X-ray backscatter imaging with bulk object transmission imaging as a further investigation in detection of illicit materials and objects in cargo items.